Sustainable sodium-ion batteries demand high-performance hard carbon (HC) anodes, yet conventional biomass pyrolysis is constrained by low carbon yield, abundant structural defects, and limited feedstock adaptability caused by violent thermal decomposition. Here, a mild methanesulfonic acid (MSA)-catalyzed dehydration strategy is reported that enables molecular-level reconstruction of biomass precursors at 80 °C, redefining the conversion of biomass to HC. Importantly, this mass-production approach eliminates compositional heterogeneity among feedstocks and is universally applicable to cotton, wood, bamboo, cellulose, glucose, cyclodextrins, and polylactic acid. The acid-catalyzed condensation transforms cellulose into oxygen-deficient cyclic aromatic ketones, which subsequently cross-link into carbon clusters. Upon pyrolysis, gas evolution (CO, CO2, CH4) is substantially suppressed, yielding a record 28.7 wt% of carbon, 59% higher than that of conventional methods. The resulting HC exhibits abundant closed pores (∼9.5 Å) and few defects, delivering a high initial coulombic efficiency (92.2%), reversible capacity of 340.6 mAh g−1, and remarkable long-term stability (96.1% capacity retention after 520 cycles/10 200 h at 0.1C). Life cycle assessment further confirms substantial reductions in energy consumption, greenhouse gas emissions, and water use compared with conventional pyrolysis. This work overcomes a key bottleneck in biomass carbon utilization, offering a scalable and high-yield dehydration route toward sodium-ion battery industrialization.